Date: September 26, 2001
To: Prof. R. Emmermann
Chairman of Executive Committee
ICDP
From: Setsuya Nakada, PI of USDP
Earthquake Research Institute, University of Tokyo
nakada@eri.u-tokyo.ac.jp
RM: Proposal submitted in January 2001, "Unzen Scientific Drilling Project (USDP) -Investigation of eruption mechanisms through scientific drilling into a volcanic conduit" (PI's (S. Nakada, K. Uto, J. C. Eichelberger, H. Shimizu, and S. Sakuma)
Your letter presented us with five points to address:
(1) Geometry of the conduit (is it reasonable to expect to intersect the conduit?),
(2) Method and techniques to penetrate the conduit (related to the first question),
(3) Methods of high-temperature core sampling and logging,
(4) Development of international science team,
And
(5) Sample circulation and data management.
1. Geometry of Conduit
The geometry of the conduit is a "dike (or plate) shaped", with the length as large as several hundred meters and the thickness of 10-20 m. Although conduits are often visualized as pipes, this is in fact a misconception. Exposed sections of arc volcanoes show that even at shallow depth the feeders are plate-like rather than pipe-like. The data on thickness and length of dikes from geological literature are summarized in Fig. 1. Based on the data, the horizontal length of the Unzen conduit is >500 m. Drilling is scheduled normally to the dike. The probability to hit the conduit is very high. As shown in Fig. 2 (same as figure 6 in our proposal), seismic tomography shows a low velocity area, which probably represents hot rock in the thickest part of the conduit, just solidified. We think the conduit dike extends from this pocket to both the east and west, an orientation indicated by surface fracturing of the summit area just prior to the onset of extrusion, and by the elongate spine that closed the eruption.
In addition, at the end of this year, we will examine the subsurface structure of the volcano summit seismologically, using vibration generators and a 3D-network with geophones at 25-m intervals. For this experiment, a multiple hydrophone system will be installed in borehole USDP-1 (Fig. 3). From this result we will determine the final target of drilling.
Fig. 1 Dimension of conduit dikes, compiled by Akira Takada. The aspect ratio of dikes depends on viscosity of magma. The length of the Unzen conduit dike (dacite) is estimated to be >500 m. Of course, one could imagine that explosive reaming could superimpose a pipe geometry on what initially was a dike, but such activity was very limited at Unzen.
Fig. 2 Seismic tomography below the summit area of Unzen volcano (Nishi et al., 2000, unpublished). Inversion was carried out using the result of a seismic experiment in 1995. Numbers represent seismic velocity (P wave) in km/sin plain view at sea level clearly indicates the low velocity region west of the summit, probably representing the thickest portion of the feeder dike, or conduit.
Fig. 3 Seismic experiment to find the precise position of the conduit, scheduled in the end of 2001. Conduit shown here represents only the thickest part.
2. Drilling Technique
The technical method and well trajectory for conduit drilling were discussed and decided by the Working Group on Technical Specifications of Conduit Drilling, which began meeting this April and included foreign scientists of GFZ and USDP as members. The latter joined in the discussions through e-mail communication.
The pilot hole drilling from the site RS-3, close to the site New RS-3 (see Fig. 4) that was carried out during January-March 2001 was very informative for preparing the plan of conduit drilling. Four-hundred-m-long slant drilling at about 45 degrees from vertical was attempted. The hole's wall stability was poor and mud-loss during the operation was too serious to continue the drilling and to maintain the angle at 45 degrees. Unfortunately, the drilling ended at 350 m slant depth and with about 54 degrees from vertical inclination for these reasons.
We compared two methods of conduit drilling from the New RS-3 site; that is, initially slanted and initially vertical drilling. In the case of slant drilling, different starting angles of 40 and 60 degrees were considered. From the scientific point of view, slant drilling with a high initial angle is attractive as a shallower level can be reached. However, slant drilling brings many risks to drillings in unstable ground and at high temperature drilling such as this project (Table 1). The points to be considered are well stability, high temperature and costs, all of which are related intimately to one another.
Slant drilling requires a round trip three times longer than normal drilling does. In the normal rig we can manage stands of 3 connected drill pipe sections (27 m-long), while the slant rig can manage only a single 9-m length (Table 1). The resulting longer operation time prevents keeping the well-bottom temperature low during drilling the hot conduit. The maximum starting diameter is smaller for slant drilling than for initially vertical drilling. This limitation decreases flexibility of the casing program to cool the well bottom effectively (refer to next chapter). Furthermore, as no slant rig for 2,000 m-class drilling exists in Japan, the cost of its lease from a foreign country should be considered; the cost estimate for transportation only may be as high as US$ 1 million.
Summarizing the above, normal drilling in which drilling starts vertically and decreases the inclination with depth is the best choice to drill the hot and challenging target with the lowest risk. The drilling trajectory and casing program we propose now are shown in Figs. 4 and 6 respectively.
Fig. 4 Well trajectories of normal (vertical) and slant drillings on the N-S cross-section of Mt. Unzen. MD = drilled depth of target, TD = terminal drilled depth.
Table 1. Comparison of drilling methods for conduit drilling at Unzen
3. High Temperature Coring and Logging
The temperature of conduit center may be as high as 600 degree C. Even by simple heat conduction model, however, such high temperatures are limited to the immediate proximity of the conduit. Seiji Saito and Nobuo Hatakeyama of our project simulated the hole bottom temperature during operation of conduit drilling (Saito and Hatakeyama, 2001). They clearly showed that the temperature is able to be controlled at low values during the operation, as a function of bottom diameter, pumping rate of mud circulation, and water inlet temperature (Fig. 5). The bottom temperature increases rapidly when round trip starts, but it decreases soon after pumping begins again. The simulated temperature does not exceed the limitations of the Positive Displacement Motor (PDM) and Measurement-While-Drilling (MWD) tools (currently about 150 degree C). According to this simulation, we are proposing the casing program shown in Fig. 6.
Fig. 5. Hole bottom temperature during operation of conduit drilling, simulated using insulated drill pipe of 3.5 W/(mK), mud flow rate of 1000 l/min., and inlet mud temperature of 20 degree C. A 8-1/2" casing was assumed from 1500m depth to the bottom. The bit lifetime, rate of penetration, speed of pull-out of the hole (POOH) and running in the hole (RIH), and operation time between POOH and RIH were 50 hours, 1 m/hour, 800 m/hour, and 4 hours, respectively. (Saito and N. Hatakeyama, 2001, Drilling Plan into the Recently Erupted Unzen Volcano and Borehole Temperature Simulations. Geothermal Resources Council Transactions, 25, 149-153.)
Fig. 6 Casing program for conduit drilling. Numbers in parenthesis are drilled depths of optional casing that was designed to reduce risk during operation.
4. National Science Team
Table 2. USDP Science Team
5. Sample Handling
Images of cores and the data of drilling conditions will be available on a web site in close to real-time, in part using DIS. As the drilling site itself is in a remote and dangerous area, visits to the rig by the public will be impossible.
Core samples will be open to scientists outside the project a year after drilling completed. The few hundred meters of core that will be recovered in the Unzen conduit-drilling project will be stored in a warehouse of either Earthquake Research Institute, University of Tokyo or Geological Survey of Japan; this will fix within a year. The right of sample distribution belongs to the PI's, as it does with other ICDP projects like the Hawaiian scientific drilling project (HSDP). We will adopt a sampling protocol similar to that used by HSDP. Any scientist who wants to conduct research on the core samples may make application to the Chief Scientist of USDP, Setsuya Nakada. A decision will be made in consultation with a panel on which all disciplines are represented.
6. Budget Plan
The budget plan is shown in Table 3. Though we have tried to reduce the total cost necessary for conduit drilling as much as possible, the present cost estimate for conduit drilling is about US$ 9.2 million (case 1 of Table 3a); slightly higher than the estimation in the proposal of this January (8.8 million). This result is due to careful re-evaluation of drilling methods both to reduce risks and to guarantee penetration of target safely. In making this evaluation, we would like to give many thanks to you for the critical comments. Japanese funding for drilling for three years will cover $ 6.2 million, as the maximum. A contribution from ICDP of $ 3 million would therefore make the project possible (case 1). If the ICDP contribution would be $ 2 million, side tracking and logging in shallow parts of the main hale would have to be deleted from the original plan (case 2).
Table 3a. Budget plan of conduit drilling
Table 3b. Total budget plan of USDP
